The present invention relates generally to anti-lock, or anti-skid, control systems for use in motor vehicles, and more particularly to such an anti-lock system which effects the anti-lock control by controlling the duty ratio of a two-position valve associated with each of wheels of the motor vehicle.
Various types of anti-lock control systems have been developed heretofore with a view to preventing a locking of a wheel of a motor vehicle which can occurs in response to a braking operation effected when the motor vehicle is running. One previous approach to resolution of this locking problem is to use a reservoir for decrease in the braking hydraulic pressure to be supplied to a wheel-braking cylinder associated with the vehicle wheel and further to use a pump for increase in the braing hydraulic pressure to be supplied thereto. There is a disadvantage which arises with this type of anti-lock system, however, in that difficulty is encountered to achieve a side-reduction and a cost-reduction due to the provision of the reservoir. Another known arrangement involves performing the decreases in the braking hydraulic pressures to the wheel braking cylinders only by using a pump without providing the reservoir. One of problems with such an anti-lock system relates to the difficulty to independently control the braking hydraulic pressures to the wheel-braking cylinders. Thus, in the case that there is a pressure difference between the braking pressures in the wheel-braking cylinders, the pressure decrease can be performed only for the high-pressure side wheel-braking cylinder but not for the low-pressure side wheel-braking cylinder. Although the independent control for the respective braking hydraulic pressures can be performed under the condition that the pump is provided for each of the wheel-braking cylinders, this results in increase in size and cost. Further, known is an anti-lock control system as disclosed in Japanese Pat. Publication No. 51-6308, the technique of which is arranged such that the braking hydraulic pressure to the wheel-braking cylinder is adjusted in accordance with duty control of a control valve having a pressure-increasing position and a pressure-decreasing position, that is, the braking hydraulic pressure is controlled so as to prevent wheel locking by varying the duty ratio in accordance with the slip ratio of the wheel. However, in this anti-lock control system, a predetermined duty ratio is used as the initial value at the time of start of the anti-lock control execution, and therefore there is presented a time lag in variation from the predetermined duty ratio to a duty ratio adequately corresponding to the vehicle-running road surface state.